Tubular polymerization reactor apparatus is used to make polyethylene, mainly by free radical initiation. Initiators may be oxygen, peroxides and similar agents. Catalysts used for coordination polymerization may also, where appropriate, be used.
The highly exothermic polymerization reaction is performed in a tubular reactor (“tube”) forming part of the apparatus under high reactor operating pressure (2000 bar to 3500 bar) under turbulent flow, at high temperatures (120° C. to 330° C.). Heat may be removed through the tube wall, which may be liquid cooled. Tubular reactors may have outputs, which vary from 50 kT to 400 kT per annum. Low cost production requires a high conversion of monomers to give as large an output of commercially desirable polymer types as is possible from given investment.
Referring to FIG. 1, a tubular polymerization reactor 100 has a tube 2 with a length typically from 200 to 1600 meters determined on the basis of the desired heat removal capacity and a diameter of from 20 to 100 mm determined on the basis of the desired capacity and the required turbulent flow.
A medium pressure, primary compressor 4, which may include a number of compressor stages, not individually shown, is connected at its intake side to a source of fresh ethylene supplied by a conduit 6, and recycled ethylene from a recycle conduit 8 at a pressure of from 20 to 70 bar. The primary compressor raises the pressure of the monomer on the outlet side to a pressure of from 250 bar to 350 bar. A high pressure, secondary compressor 5, which may include a number of compressor stages, is connected at its intake side to the outlet side of the primary compressor 4 and raises the pressure of the feed containing ethylene further to the reactor operating pressure as indicated above of from 2000 to 3500 bar. The compressed pressurized monomer is then fed through conduits 12, 14 to various monomer feed locations 3 spaced lengthwise along tube 2.
Multiple free-radical initiator or catalyst injection positions 7 are also spaced lengthwise of the tube 2 to cause the monomer to be converted into polymer in at least two reaction zones formed inside the tube 2.
A mixture of polymer and unreacted monomer formed in the tube 2 passes from tube outlet 16 to a separating and recycling part of the polymerization apparatus. This part includes a high-pressure separator (HPS) 18, which receives the monomer-polymer mixture from the outlet of the tube 2. The HPS is connected to convey a mixture of polymer and monomer produced, to a low-pressure separator (LPS) 20 for further monomer removal. The resulting molten polymer phase is passed from the LPS 20 to a polymer finishing section with an extruder 22. A volatile monomer-rich phase comprising unreacted monomer separated in HPS 18, passes through a recycle conduit 24 at a pressure of approximately that of the outlet of the primary compressor 4 through line 26 to join the monomer containing feed passing from the primary to the secondary compressor 5. The volatile monomer rich phase including unreacted monomer from the LPS 20 passes to a low pressure purge compressor 21, which may have a number of stages, at a pressure above that at the intake of the primary compressor to the intake of the primary compressor 4.
At some location in the circuit a chain transfer agent is added for supply to the tube 2. Transfer agents are used to reduce the molecular weight, which can be expressed in a melt index (MI) value, and to narrow the molecular weight distribution (MWD).
A typical product range is shown in FIG. 2 and covers a melt index (“M”, I2.16) of from 0.1 to 50 dg/min, a molecular weight distribution (MWD) of from 5 to 50 and a haze of from 1 to 20. Haze is determined by ASTM D-1003; MI is determined by ASTM-1238 Condition E; Mw and Mn were measured by GPC (Gel Permeation Chromatography) on a Waters 150 gel permeation chromatograph equipped with a differential refractive index (DRI) detector and a Chromatix KMX-6 on line light scattering photometer. The system was used at 135° C. with 1,2,4-trichlorobenzene as the mobile phase. Shodex (Showa Denko America, Inc) polystyrene gel columns 802, 803, 804 and 805 were used. This technique is discussed in “Liquid Chromatography of Polymers and Related Materials III”, J. Cazes editor, Marcel Dekker. 1981, p. 207, which is incorporated herein by reference. No corrections for column spreading were applied; however, data on generally accepted standards, e.g., National Bureau of Standards Polyethylene 1484, and anionically produced hydrogenated polyisoprenes (an alternating ethylene-propylene copolymer) demonstrated that such corrections on Mw/Mn (=MWD) were less than 0.05 units. Mw/Mn was calculated from elution times. The numerical analyses were performed using the commercially available Beckman/CIS customized LALLS software in conjunction with the standard Gel Permeation package. Calculations involved in the characterization of polymers by 13CNMR follow the work of F. A. Bovey in “Polymer Conformation and Configuration” Academic Press, New York, 1969.
In practical use of the apparatus, product quality has to be balanced with desired production economics. Higher conversion (giving low energy and recycle costs) tends to lead to a broader MWD and significant branching which leads too high and unacceptable haze values. Low density polyethylene requires production of relatively many short chain branches. Olefinically unsaturated comonomers are then used which have a low transfer coefficient (efficiency of transfer agents) and hence little chain length reducing activity. Examples are propylene or butene-1. A high concentration of such comonomers is needed to achieve a desired melt index, restricting the productive capacity on a given reactor. In some cases, certain areas of theoretically available MI, haze and density combinations cannot be produced at an acceptable cost. Particularly narrow molecular weight distribution (MWD), relatively high density polyethylenes generally cannot be made economically with saturated alkane transfer agents (which do not incorporate in the chain) as they have a very low transfer constant, lower than the propylene and butene-1 used for low density polyethylenes.
An initiator or catalyst injection position is associated with each reaction zone. Injection of the initiator causes an exothermic temperature rise which is removed by a cooling at the zone and downstream of that zone. The cooling is effected through the tube wall, optionally aided by a cooling liquid as a heat transfer medium and/or by a feed of cold monomer that is added downstream. Further, initiator may be added downstream to form another reaction zone for converting additional monomer into polymer.
Generally speaking, in the prior art, transfer agents have been added so as to have roughly the same concentration of chain transfer agent in each monomer feed. From an apparatus point of view, this can be achieved by mixing the transfer agent with the monomer fed before the monomer is compressed by the secondary compressor. The transfer agent is then added equally along the length of the tube, although it may be consumed unequally and so concentration variations along the tube may result.
In FIG. 1, a source 23 of transfer agent is connected to the intake of the primary compressor 4 and hence distributed, after passing through the secondary compressor 5, to the different monomer feeds 3 spaced along the tube 2. The recycle stream 8 coming from the LPS 20 and the purge compressor 21 is also passed to the intake of primary compressor 4. The recycle from the HPS 18 which contains unconsumed transfer agent is passed to the intake of the secondary compressor. Thus the transfer agent and monomer form a single, common gas stream with the desired concentration of transfer agent for compression in the secondary compressor 5 and for supply to the various feed positions 3 along the tube 2.
Furthermore, by selecting a transfer agent which has a low chain transfer activity, higher concentrations of transfer agent have to be used in the non-polymer gaseous fraction of the tube contents to achieve a target MI. The low chain transfer activity contributes to the creation of small transfer agent concentrations along the length of the tube where the chain transfer agents also have a low reactivity ratio. By using unsaturated transfer agents with low chain transfer activity, branches are formed along the polymer backbone and the density of the resulting polymer is reduced. In such apparatus, mostly chain transfer agents have been used having a chain transfer constant of less than 0.03.
The recycle from the HPS and LPS still contain, for transfer agents with a low reactivity ratio, a high level of transfer agent and the amount that is added from the source 23 is low relative to that present in the recycle 26 and 8.
It would be desirable to have methods and apparatus for transfer agent addition and selection so as to increase the process efficiency while obtaining a satisfactory commercial product, or to produce more satisfactory commercial products at prevailing process efficiencies.